Domain alias resolution

In certain regions, Google operated googlemail.com as an alias for gmail.com — alice@googlemail.com and alice@gmail.com are the same mailbox. Without alias resolution, the same person can appear as two separate contacts, receiving duplicate sends and having their unsubscribe applied only to the alias variant. While googlemail.com is the most common example, additional alias relationships exist for other providers in specific markets. Duplicate contacts from unresolved aliases inflate list metrics, add send cost, and introduce suppression gaps when the unsubscribe is captured under the aliased domain.

Apply a domain alias map during normalization, resolving aliased domains to their canonical form before deduplication lookup:

const DOMAIN_ALIASES: Record<string, string> = {
  'googlemail.com': 'gmail.com',
  'googlemail.co.uk': 'gmail.com',
}

function normalizeEmail(email: string): string {
  const [local, domain] = email.toLowerCase().trim().split('@')
  if (!local || !domain) return email.toLowerCase().trim()
  const canonicalLocal = local.split('+')[0]
  const canonicalDomain = DOMAIN_ALIASES[domain] ?? domain
  return `${canonicalLocal}@${canonicalDomain}`
}

Combine this with plus-tag stripping (see ab-000860) in a single normalizeEmail function so all normalization logic runs in one pass. Add new alias entries as you discover them via duplicate detection reports.